Question
We are trying to move away from urea resin glue for hot pressing. There are a few PVA glues for hot pressing. Some are pre-catalyzed and some are post catalyzed. There are also quite a few manufacturers out there. I would like to hear from people who have tried these glues and what their experience has been.
Forum Responses
(Veneer Forum)
From contributor A:
With all due respect, may I ask why? Urea has much lower cost, much higher heat resistance and higher moisture resistance. True urea is darker in color but it sands easier and cleaner.
We also use a lot of Wonderbond (Borden) in the hot press for bendable 2-ply and skins. It's a PVA and set up very fast in the press. It also is good for re-pressing as in step pressing etc. We like to UF glue whenever possible for the reasons mentioned above.
What are your concerns with PVA? The biggest problem is that it adds more moisture to the panel and has to sit longer before machining. Also, do you hand roll your glue? We use a BB roller coater and use much less of a spread compared to hand rolling. What kind of press do you use? How many SF of panels do you press per run?
Type I testing involves cutting the 6" by 6" assemblies into 1" by 3" specimens, boiling them for 4 hours, then baking the specimens in a 145°F oven for 20 hours. They are boiled for an additional 4 hours, then immediately cooled using running water. The specimens are sheared while wet, and the bonds must pass certain strength and wood failure requirements to pass the Type I specification.
Type II testing involves cutting the 6" by 6" assemblies into 2" by 5" specimens, soaking them for 4 hours, then baking the specimens in a 120°F oven for 19 hours. This is repeated for a total of three cycles, and the bonds must not delaminate to pass the Type II specification.
To simplify, Type I is waterproof, i.e. Resorcinol for building boats, kayaks, canoes. Type II is water resistant - urea for plywood exposed to the elements but not particularly in direct contact with water - soaked or in water for extended periods of time.
Cite: HPVA study
LOW FORMALDEHYDE EMITTING HARDWOOD PLYWOOD
Hardwood plywood wood products are available today which emit 60-90% less formaldehyde than products manufactured in the early to mid 1980's. In the United States there is one federal standard which sets a limit on formaldehyde emissions from wood products. This standard was developed in 1985 by the U.S. Department of Housing and Urban Development (HUD) for plywood, particleboard, and industrial panels used in manufactured (mobile) homes, and the limits were established as follows:
Plywood Wall Panels 0.2 ppm @ 0.29 ft2/ft3 Loading Rate
Particleboard, Industrial Hardwood Plywood Panels 0.3 ppm @ 0.13 ft2/ft3 Loading Rate
The 0.2 ppm and 0.3 ppm levels are based on a large chamber test method which specifies the parameters of test such as product loading rate, temperature, relative humidity, and air change rate. One state standard (Minnesota) and industry standards have also been developed which contain requirements similar to those covered in the HUD standard. It should be noted that the loading rates used in testing are significantly higher than the loading rate found in most homes. Therefore, the formaldehyde concentration found in a home would be expected to be considerably less than the 0.2 or 0.3 ppm maximum limits established by the standards.
Now let's compare these emissions to other indoor formaldehyde pollutants:
Cite: Calif. EPA study
Formaldehyde Emissions from Selected Indoor Sources
Wet products
Wood floor finish 11,000
Fingernail hardener 300
Latex paint 9
Wood products
Cabinet door with acid-cured finish 460
Medium-density fiberboard cabinet door 360
Particle board 240
Particle board with vinyl laminate 16
Softwood plywood 4
Other
New permanent-press shirts 10
Washed permanent-press shirts 42
Fiberglass insulation 32
There is exposure to off gassing from a wide variety of sources, many with higher concentrations than those involving the use of urea glue lines. I would be interested in knowing if there is a greater exposure to urea as a result of ripping plywood on a table saw as opposed to hot pressing with Urea resin adhesives!
Comment from contributor G:
Our operation that focuses on high end office furnishings originally employed UF resins. We gradually (for the sake of not wanting to deal with the issues associated with UF resins like off gassing and waste disposal) moved our operation to PVA adhesives in all areas. This includes edging, hot pressing, RF curing operations and edge gluing as well as assembly work.
Now, we are actually getting back into UF resins simply because they far out perform PVA's. We will still likely keep PVA's in use with our hot pressing operation (despite the issues of checking etc. that can occur). The fact of the matter is that no PVA will replace the bonding properties of UF resins, save that of PUR's which have their own issues. What prompted our shift was the fact that we were experiencing a tremendous amount of creep in laminated headers on curved surfaces. Some of these headers are made up of 10 4mm plys, so you can imagine how terrible the finish could look on a top surface when the plys began to creep slightly! Currently, we are using National Casein 705UR.
